1. Field of the Invention
The present invention relates to the improvement of a control apparatus for the traction of rolling stock which serves to effectually utilize the adhesive force (frictional force) between wheels of the rolling stock and rails as the tractive or braking force.
2. Description of the Related Art
It is well known that, since vehicles on rails obtain their tractive or braking force by virtue of the friction between the wheels thereof and the rails, slipping or skidding of the wheels will occur if the driving or braking torque goes beyond a limited value (a maximal adhesive force) defined by the coefficient of friction between the wheels and the rails. The slip occurring during power running is based on substantially a same phenomenon as the skid occurring at the time of braking. Thus, it should be possible to employ a single means as a way of preventing both. Accordingly, hereinafter, a description will be presented mainly based on an example of the operation in the power running of an electric vehicle, the especially different characteristics relative to the braking period will be described at the time needed.
Conventionally, various measures have been proposed in order to improve a re-adhesion control or traction recovery control, in which the nonadhesive state such as slip of driving wheels of an electric vehicle is removed by swiftly reducing the driving torque applied to the wheels when such state was detected. The conventional measures have an effect in the recovery of the nonadhesive state, however they have resulted in the fact that the driving torque or the braking force has been apt to be reduced too much for preventing the recurrence of nonadhesive state just after the recovery, and therefore the maximal adhesive force could not be effectively utilized for the tractive or braking force.
By the way, the following is known about the relationship of the traction force f with respect to the relative velocity v.sub.s between the peripheral velocity v.sub.m of a driving wheel and vehicle velocity v.sub.t. The relative velocity v.sub.s (=.vertline.vm-v.sub.t .vertline.), which is usually called a creep velocity, increases as the traction force f increases with the driving torque. By further increasing the driving torque, the traction force f reaches its maximal value f.sub.max, which is determined by the product .mu.W of the friction coefficient .mu. between the driving wheel and the surface of the rail and the axle weight W of the driving wheel. Further, the maximal traction force f.sub.max is equivalent to the aforesaid maximal adhesive force, which depends on the surface condition of the rail to a great extent.
After that, even though the driving torque is increased, the traction force f decreases, resulting in that only the creep velocity v.sub.s increases. In this manner, in the relationship of the traction force f versus the creep velocity v.sub.s, there exists a specific creep velocity v.sub.so at which the traction force f becomes maximal. In the region of the creep velocity lower than v.sub.so, the traction force f increases with the creep velocity v.sub.s, and in the region of the creep velocity higher than v.sub.so the traction force f decrease with increase of the creep velocity v.sub.s. Further, it is to be noted here that the existence of the creep velocity v.sub.s, even if it is very small, means the existence of slip. Notwithstanding, the former region is not usually said to belong to the nonadhesive state. Such a status can be always observed during acceleration or deceleration of a vehicle, i.e., except the case where a vehicle continues to run at a constant velocity. Therefore, the former region is called the condition of a pseudo slip.
Then, noting the relationship of the traction force f versus the creep velocity v.sub.s as mentioned above, some of the applicants have proposed an improved system which can exhaustively utilize the maximal adhesive force for the tractive or braking force (U.S. patent application Ser. No. 820,327 filed Jan. 21, 1986, now U.S. Pat. No. 4,701,682). According thereto, the changing rate .DELTA.v.sub.s /.DELTA.t of the creep velocity v.sub.s with respect to time is obtained. Further, the changing rate .DELTA.f/.DELTA.t of the traction force f produced in the driving wheel at that time is obtained from the armature current of a main driving motor. The driving torque produced by the main motor is so controlled as to be decreased when polarities of the changing rate .DELTA.v.sub.s /.DELTA.t in the creep velocity and that .DELTA.f/.DELTA.t in the traction force are different from each other.
Namely, in the prior proposal mentioned above, by monitoring the polarities of these two changing rates, it is discriminated whether or not the driving torque produced by the main motor at that time causes the traction force f to exceed the maximal adhesive force f.sub.max (=.mu.W). In other words, the traction control is conducted in such a way that the traction force f and the creep velocity v.sub.s are maintained at the maximal adhesive force f.sub.max and the aforesaid specific creep velocity v.sub.so, respectively. Consequently, the excellent adhesion capability can be achieved. In the above mentioned prior proposal, however, the considerably large amount of arithmetic logic operation is required in order to obtain various control variables and discriminate the control status, so that a large scale of processing apparatus has become necessary for exclusive use in such arithmetic logic operation.